Adds iiif.io theme and showcase

- Added styles and content from main iiif.io site

- Showcase items are rendered with data from `_data/showcase.yml` file. 

- Individual preview/embed pages (`Learn more` targets) are also driven by `_data/showcase.yml`. This can be changed by augmenting current data file and/or by having separate data files for each showcase item.

- To add a new showcase item - 
  - Drop new thumbnail image to `source/img/showcase` folder
  - Append entry to `_data/showcase.yml` with relevant data

![image](https://cloud.githubusercontent.com/assets/302258/4535003/59946aae-4dac-11e4-8e45-781972396edb.png)
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![image](https://cloud.githubusercontent.com/assets/302258/4535058/4864dc36-4dad-11e4-8e4a-d900ee7cc984.png)